Fuel cells which generate electric current by controllably combining elemental hydrogen and oxygen are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are separated by a permeable electrolyte formed of a ceramic solid oxide. Such a fuel cell is known in the art as a “solid-oxide fuel cell” (SOFC). Hydrogen, either pure or reformed from hydrocarbons, is flowed along the outer surface of the anode and diffuses into the anode. Oxygen, typically from air, is flowed along the outer surface of the cathode and diffuses into the cathode. Each O2 molecule is split and reduced to two O−2 ions catalytically by the cathode. The oxygen ions diffuse through the electrolyte and combine at the anode/electrolyte interface with four hydrogen ions to form two molecules of water. The anode and the cathode are connected externally through the load to complete the circuit whereby four electrons are transferred from the anode to the cathode. When hydrogen is derived by “reforming” hydrocarbons such as gasoline in the presence of limited oxygen, the “reformate” gas includes CO which is converted to CO2 at the anode. Reformed gasoline is a commonly used fuel in automotive fuel cell applications.
A complete SOFC system typically includes auxiliary subsystems for, among other requirements, generating fuel by reforming hydrocarbons; tempering the reformate fuel and air entering the stack; providing air to the hydrocarbon reformer; providing air to the cathodes for reaction with hydrogen in the fuel cell stack; providing air for cooling the fuel cell stack; providing combustion air to an afterburner for unspent fuel exiting the stack; and providing cooling air to the afterburner and the stack.
An enclosure for an SOFC system has two basic functions. The first function is to provide thermal insulation for some of the components which must function at an elevated temperature (700° C.–900° C.) to maintain them at that temperature for efficient operation, to protect lower temperature components outside the thermal enclosure, and to reduce the exterior temperature over the overall unit to a human-safe level. The second function is to provide structural support for mounting of individual components, mounting the system to another structure such as a vehicle, protection of the internal components from the exterior environment, and protection of the surrounding environment from the high temperatures of the fuel cell assembly.
In a solid-oxide fuel cell system, the “hot” components, e.g., the fuel cell stacks, the fuel reformer, tail gas combuster, heat exchangers, and fuel/air manifold, are contained in a “hot zone” within the thermal enclosure. The thermal enclosure is intended specifically for minimizing heat transfer to its exterior and has no significant structural or protective function for its contents. A separate and larger structural enclosure surrounds the thermal enclosure, defining a “cool zone” outside the thermal enclosure for incorporation of “cool” components, e.g., the air supply system and the electronic control system. The structural enclosure components are known in the art as a “plant support module” (PSM).
Process air entering the blower fan in the PSM first enters the enclosure generally through a filter and then is drawn into the blower, passing as cooling air around the electronics process control module (ECM) and the blower motor. The air cools these components and is desirably warmed thereby before being directed to the fuel cell reformer and stacks via a manifold or plenum having a plurality of independently-controllable air valves for metering air as needed to a plurality of process locations and functions. Thus, the PSM is constantly purged through the downstream processes.
When the fuel cell system is shut down, the enclosure may accumulate harmful gases which then are collected by the blower and discharged through the fuel cell system during the next startup, which discharge may be damaging to the fuel cells.
What is needed is means for preventing the discharge of gases in the PSM enclosure into the fuel cell system immediately upon startup thereof.
It is a principal object of the present invention to reduce the risk of damage to a fuel cell system from harmful gases at start-up.
It is a further object of the invention to increase the reliability and safety of operation of such a fuel cell system.